We have shown that the Mouse Mammary Tumor Virus (MMTV) long terminal repeat (LTR) contains the target site for steroid hormone action associated with this virus. Subsequently, deletion analysis of MMTV LTR has indicated that approximately 100 out of the 1300 bp of the LTR mediate hormone regulation of transcription. However, while these experiments define the region of MMTV that is necessary for steroid hormone regulation of gene expression, they do not directly address the molecular mechanisms by which hormone-receptor complex, MMTV LTR, RNA polymerase II, and other nuclear protein components interact to produce the observed biological phenomenon of elevated rates of transcription. In order to begin to understand this complex problem, we feel that reconstitution of hormone regulated transcription in vitro will be required. Reconstitution attempts using current state-of-the-art technology have failed, most probably because they do not assemble the correct epigenetic structure. To overcome this problem, we are trying to introduce MMTV LTR into eukaryotic cells as an episome, by using the Bovine Papilloma Virus (BPV) as a vector. Such an approach will allow us to isolate MMTV LTR as nucleoprotein minichromosomes with their in vivo chromatin structure intact.